Nonvolatile semiconductor storage device with charge storage layer and its manufacturing method

ABSTRACT

A nonvolatile semiconductor storage device includes a semiconductor substrate, a charge storage layer formed above the semiconductor substrate, a control gate formed above the charge storage layer, a silicide layer formed above the control gate, a word gate formed above a side of the control gate. A top surface of the silicide layer is flat.

CROSS-REFERENCE TO RELATED APPLICATION

The present application is related to U.S. patent application Ser. No.______, concurrently filed herewith, and which is based on JapanesePatent Application No. 2008-055597 filed on Mar. 5, 2008.

INCORPORATION BY REFERENCE

This application is based upon and claims the benefit of priority fromJapanese Patent Application No. 2008-055598 which was filed on Mar. 5,2008, the disclosure of which is incorporated herein in its entirety byreference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a nonvolatile semiconductor storagedevice and to a method of manufacturing the same, and more particularlyto a charge trap-type nonvolatile semiconductor storage device and to amethod of manufacturing the same.

2. Description of Related Art

As the nonvolatile semiconductor storage device, there has been known acharge trap-type nonvolatile semiconductor storage device. For example,JP-A-2004-312009 (corresponding U.S. Pat. No. 7,005,349B2) discloses amethod of manufacturing a silicon-oxide-nitride-oxide-silicon (SONOS)memory device.

FIGS. 1 to 9 are cross-sectional views showing the SONOS memory devicemanufacturing method in JP-A-2004-312009. First, as shown in FIG. 1, adielectric layer 111 made of ONO (oxide-nitride-oxide) is formed on asemiconductor substrate 110. Then, a first conductive layer 130 a isformed on the dielectric layer 111. Thereafter, a buffer layer 180 isformed on the first conductive layer 130 a. Then, as shown in FIG. 2, atrench 181 from which a partial surface of the first conductive layer130 a is exposed is formed on the buffer layer 180. Subsequently, asshown in FIG. 3, a first insulating film 117 a is so formed as to coverthe trench 181.

Then, as shown in FIG. 4, the first insulating film 117 a is etched backto form a first insulating spacer 117 on an inner wall of the trench181. Thereafter, as shown in FIG. 5, an exposed portion of the firstconductive layer 130 a and a portion of the dielectric layer 111therebelow are selectively sequentially removed with the firstinsulating spacer as an etching mask, thereby dividing the firstconductive layer 130 a and the dielectric layer 111 into two portions,respectively.

Subsequently, as shown in FIG. 6, a gate dielectric layer 115 is formedon the semiconductor substrate 110 exposed by separation of thedielectric layer 111. The gate dielectric layer 115 extends on the firstinsulating spacer 117 so as to insulate the two first conductive layers130 a from each other so that the two separated first conductive layers130 a below the first insulating spacers 117 are allowed to function asindependent gates (control gates 130), respectively. Subsequently, asecond conductive layer 120 (word gate 120) embedded in a gap betweenboth side walls of the trench is formed on the gate dielectric layer115. Then, a capping insulating layer 118 is so formed as to cover theupper portion of the second conductive layer 120.

Then, as shown in FIG. 7, the buffer layer 180 is removed with the firstinsulating spacer 117 as an etching mask. Subsequently, as shown in FIG.8, a portion of the first conductive layer 130 a which has been exposedby removal of the buffer layer 180, and a portion of the dielectriclayer 111 therebelow are selectively sequentially removed with the firstinsulating spacer 117 as an etching mask to provide the dielectric layer111 and the first conductive layer 130 (control gate 130) which havebeen divided into two parts, respectively, as a final pattern.

Then, as shown in FIG. 9, a first diffusion layer 151 a is formed on thesemiconductor substrate 110 exposed outside of the final pattern(dielectric layer 111) by ion implantation. Then, a second insulatingspacer 116 is formed on the side walls of the dielectric layer 111 andthe first conductive layer 130 which are the final pattern. Then, asecond diffusion layer 151 b is formed on the semiconductor substrate110 by ion implantation with the second insulating spacer 116 as a mask.Thereafter, although being not shown, a second silicide layer isselectively formed on the second diffusion layer 151 b, and a thirdsilicide layer is formed on the second conductive layer 120 in asilicide inducing process. The first conductive layer and the secondconductive layer are each formed to include a conductive silicon layer.

Also, T. Saito et al., “Hot hole erase characteristics and reliabilityin twin MONOS device”, IEEE non-volatile semiconductor memory workshop,pp. 50-52, 2003 discloses a device of a twin-MONOS structure as anonvolatile semiconductor memory device of a split gate type.

SUMMARY

With the advanced miniaturization of a memory, a demand has been made toincrease an operation speed. However, in a technique (FIGS. 1 to 9) ofJP-A-2004-312009 described above, because the first insulating spacer117 exists on the first conductive layer 130 (control gate 130), it isimpossible to turn polysilicon as the first conductive layer 130 intosilicide. For that reason, the control gate electrode becomes high inresistance, and obstructs the high speed operation. A technique thatreduces the resistance of the control gate electrode to enable the highspeed operation is desired.

The present invention seeks to solve one or more of the above problems,or to improve upon those problems at least in part.

In one exemplary embodiment, a nonvolatile semiconductor storage deviceaccording to the present invention includes a semiconductor substrate, acharge storage layer formed above the semiconductor substrate, a controlgate formed above the charge storage layer, a silicide layer formedabove the control gate and a word gate formed above a side of thecontrol gate. A top surface of the silicide layer is substantially flat.

In another exemplary embodiment, a nonvolatile semiconductor storagedevice according to the present invention includes a semiconductorsubstrate, a first charge storage layer formed above the semiconductorsubstrate, a second charge storage layer formed above the semiconductorsubstrate, a word gate formed between the first charge storage layer andthe second charge storage layer, a first control gate formed above thefirst charge storage layer, a second control gate formed above thesecond charge storage layer, a first silicide layer formed above thefirst control gate and a second silicide layer formed above the secondcontrol gate. A top surface of the silicide layer is substantially flat.

Thus, in the nonvolatile semiconductor storage device according to thepresent invention, the silicide layer whose top surface is substantiallyflat is formed on the control gate. The silicide layer enables thecontrol gate to be reduced in resistance. As a result, it is possible toperform the high speed operation of this nonvolatile semiconductorstorage device.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other purposes, advantages and features of the presentinvention will become more apparent from the following description ofcertain exemplary embodiments taken in conjunction with the accompanyingdrawings in which:

FIG. 1 is a cross-sectional view showing a manufacturing method ofJP-A-2004-312009.

FIG. 2 is a cross-sectional view showing a manufacturing method ofJP-A-2004-312009.

FIG. 3 is a cross-sectional view showing a manufacturing method ofJP-A-2004-312009.

FIG. 4 is a cross-sectional view showing a manufacturing method ofJP-A-2004-312009.

FIG. 5 is a cross-sectional view showing a manufacturing method ofJP-A-2004-312009.

FIG. 6 is a cross-sectional view showing a manufacturing method ofJP-A-2004-312009.

FIG. 7 is a cross-sectional view showing a manufacturing method ofJP-A-2004-312009.

FIG. 8 is a cross-sectional view showing a manufacturing method ofJP-A-2004-312009.

FIG. 9 is a cross-sectional view showing a manufacturing method ofJP-A-2004-312009.

FIG. 10 is a cross-sectional view showing the configuration of anonvolatile semiconductor storage device according to a first exemplaryembodiment of the present invention.

FIG. 11 is a cross-sectional view showing the configuration of anonvolatile semiconductor storage device according to the firstexemplary embodiment of the present invention.

FIG. 12 is a cross-sectional view showing a manufacturing method for anonvolatile semiconductor storage device according to the firstexemplary embodiment of the present invention.

FIG. 13 is a cross-sectional view showing a manufacturing method for anonvolatile semiconductor storage device according to the firstexemplary embodiment of the present invention.

FIG. 14 is a cross-sectional view showing a manufacturing method for anonvolatile semiconductor storage device according to the firstexemplary embodiment of the present invention.

FIG. 15 is a cross-sectional view showing a manufacturing method for anonvolatile semiconductor storage device according to the firstexemplary embodiment of the present invention.

FIG. 16 is a cross-sectional view showing a manufacturing method for anonvolatile semiconductor storage device according to the firstexemplary embodiment of the present invention.

FIG. 17 is a cross-sectional view showing a manufacturing method for anonvolatile semiconductor storage device according to the firstexemplary embodiment of the present invention.

FIG. 18 is a cross-sectional view showing a manufacturing method for anonvolatile semiconductor storage device according to the firstexemplary embodiment of the present invention.

FIG. 19 is a cross-sectional view showing a manufacturing method for anonvolatile semiconductor storage device according to the firstexemplary embodiment of the present invention.

FIG. 20 is a cross-sectional view showing a manufacturing method for anonvolatile semiconductor storage device according to the firstexemplary embodiment of the present invention.

FIG. 21 is a cross-sectional view showing a manufacturing method for anonvolatile semiconductor storage device according to the firstexemplary embodiment of the present invention.

FIG. 22 is a cross-sectional view showing a manufacturing method for anonvolatile semiconductor storage device according to the firstexemplary embodiment of the present invention.

FIG. 23 is a cross-sectional view showing a manufacturing method for anonvolatile semiconductor storage device according to the firstexemplary embodiment of the present invention.

FIG. 24 is a cross-sectional view showing a manufacturing method for anonvolatile semiconductor storage device according to the firstexemplary embodiment of the present invention.

FIG. 25 is a cross-sectional view showing a manufacturing method for anonvolatile semiconductor storage device according to the firstexemplary embodiment of the present invention.

FIG. 26 is a cross-sectional view showing a manufacturing method for anonvolatile semiconductor storage device according to the firstexemplary embodiment of the present invention.

FIG. 27 is a cross-sectional view showing a manufacturing method for anonvolatile semiconductor storage device according to the firstexemplary embodiment of the present invention.

FIG. 28 is a cross-sectional view showing a manufacturing method for anonvolatile semiconductor storage device according to the firstexemplary embodiment of the present invention.

FIG. 29 is a cross-sectional view for explaining problems of a techniqueof JP-A-2004-312009.

FIG. 30 is a cross-sectional view for explaining the advantages of thenonvolatile semiconductor storage device and the manufacturing methodtherefor according to the first exemplary embodiment of the presentinvention.

FIG. 31 is a cross-sectional view showing a manufacturing process whenan upper configuration of a word gate electrode is spread in themanufacturing method for the nonvolatile semiconductor storage deviceaccording to the first exemplary embodiment of the present invention.

FIG. 32 is a cross-sectional view showing a manufacturing process whenan upper configuration of a word gate electrode is spread in themanufacturing method for the nonvolatile semiconductor storage deviceaccording to the first exemplary embodiment of the present invention.

FIG. 33 is a cross-sectional view showing a manufacturing process whenan upper configuration of a word gate electrode is spread in themanufacturing method for the nonvolatile semiconductor storage deviceaccording to the first exemplary embodiment of the present invention.

FIG. 34 is a cross-sectional view showing a manufacturing process whenan upper configuration of a word gate electrode is spread in themanufacturing method for the nonvolatile semiconductor storage deviceaccording to the first exemplary embodiment of the present invention.

FIG. 35 is a cross-sectional view showing a manufacturing process whenan upper configuration of a word gate electrode is spread in themanufacturing method for the nonvolatile semiconductor storage deviceaccording to the first exemplary embodiment of the present invention.

FIG. 36 is a cross-sectional view showing the configuration of anonvolatile semiconductor storage device according to a second exemplaryembodiment of the present invention.

FIG. 37 is a cross-sectional view showing a manufacturing method for thenonvolatile semiconductor storage device according to the secondexemplary embodiment of the present invention.

FIG. 38 is a cross-sectional view showing a manufacturing method for thenonvolatile semiconductor storage device according to the secondexemplary embodiment of the present invention.

FIG. 39 is a cross-sectional view showing a manufacturing method for thenonvolatile semiconductor storage device according to the secondexemplary embodiment of the present invention.

FIG. 40 is a cross-sectional view showing a manufacturing method for thenonvolatile semiconductor storage device according to the secondexemplary embodiment of the present invention.

FIG. 41 is a cross-sectional view showing a manufacturing method for thenonvolatile semiconductor storage device according to the secondexemplary embodiment of the present invention.

FIG. 42 is a cross-sectional view showing a manufacturing method for thenonvolatile semiconductor storage device according to the secondexemplary embodiment of the present invention.

FIG. 43 is a cross-sectional view showing a manufacturing method for thenonvolatile semiconductor storage device according to the secondexemplary embodiment of the present invention.

FIG. 44 is a cross-sectional view showing a manufacturing method for thenonvolatile semiconductor storage device according to the secondexemplary embodiment of the present invention.

FIG. 45 is a cross-sectional view showing a manufacturing method for thenonvolatile semiconductor storage device according to the secondexemplary embodiment of the present invention.

FIG. 46 is a cross-sectional view showing a manufacturing method for thenonvolatile semiconductor storage device according to the secondexemplary embodiment of the present invention.

FIG. 47 is a cross-sectional view showing a manufacturing method for thenonvolatile semiconductor storage device according to the secondexemplary embodiment of the present invention.

DETAILED DESCRIPTION OF THE EXEMPLARY EMBODIMENTS

The invention will now be described herein with reference toillustrative exemplary embodiments. Those skilled in the art willrecognize that many alternative embodiments can be accomplished usingthe knowledge of the present invention and that the invention is notlimited to the exemplary embodiments illustrated for explanatorypurposes.

First Exemplary Embodiment

First, a description will be given of the configuration of a nonvolatilesemiconductor storage device according to the first exemplary embodimentof the present invention. FIG. 10 is a cross-sectional view showing theconfiguration of the nonvolatile semiconductor storage device accordingto the first exemplary embodiment of the present invention. FIG. 10exemplifies a flash memory cell with a TWIN-MONOS structure as a memorycell 2 of the nonvolatile semiconductor storage device according to thefirst exemplary embodiment.

The memory cell 2 includes a word gate electrode 20, a word gateinsulating film 15, a control gate electrode 30, an Oxide Nitride Oxidefilm: oxide-nitride-oxide (ONO) film 11, a side wall insulating film 16,a source/drain diffusion layer 51, and silicide layers 61, 62, 63.

The word gate electrode 20 is formed on a channel region (a surfaceregion of the semiconductor substrate 10) through the word gateinsulating film 15. The word gate electrode 20 is exemplified bypolysilicon doped with impurities. A height of the word gate electrode20 from the surface of the semiconductor substrate 10 is higher than aheight of the control gate electrode 30 from the surface of thesemiconductor substrate 10. As a result, when the silicide layers 62 and63 (which will be described later) are formed on the word gate electrode20 and the control gate electrode 30, there is no risk that a layer thatshort-circuits the word gate electrode 20 (silicide layer 62) and thecontrol gate electrode 30 (silicide layer 63) is formed. The word gateelectrode 20 may have a top surface side larger in width than a bottomsurface side as will be described later. In such a case, since an areaof the silicide layer 62 can be widened, the configuration cancontribute to low resistance of the silicide layer 62. The silicidelayer 62 is formed on an upper portion of the word gate electrode 20.The silicide layer 62 is exemplified by a cobalt silicide.

The word gate insulating film 15 is formed on the channel regionsandwiched between the source/drain diffusion layers 51 so as to cover abottom surface and both side surfaces of the word gate electrode 20. Theword gate insulating film 15 is exemplified by silicon oxide. The wordgate insulating film 15 has no function of storing electric charges.That is, no electric charges are stored on the bottom surface and theside surfaces of the word gate electrode 20. No ONO film 11 (chargestorage layer: described later) is located at the side surfaces of theword gate electrode 20. Instead, the word gate insulating film 15 coversthe side surfaces of the word gate electrode 20, thereby making itpossible to set a charge storage region to only the ONO film 11 belowthe control gate electrode 30. As a result, electric charges can locallyexist in only the ONO film 11 facing the channel region, therebyincreasing the reliability of the operation.

The control gate electrodes 30 are formed on both side surfaces of theword gate electrode 20 through the word gate insulating film 15, and onthe channel region through the ONO films 11. The control gate electrodes30 are exemplified by polysilicon doped with impurities. The topsurfaces of the control gate electrodes 30 are substantially parallel tothe surface of the semiconductor substrates 10, and are substantiallyflat. In the TWIN-MONOS structure shown in FIG. 10, one memory cell 2has the two control gate electrodes 30 on both sides of one word gateelectrode 20. The silicide layers 63 are formed on the control gateelectrodes 30. The silicide layers 63 are exemplified by cobaltsilicide. The top surfaces of the silicide layers 63 are substantiallyparallel to the surface of the semiconductor substrate 10 and the topsurfaces of the control gate electrodes 30, and flat.

Each of the ONO films 11 is a charge storage layer, and formed betweenthe control gate electrode 30 and the channel region. The ONO film 11may have a three-layer structure including an oxide film 12, a nitridefilm 13, and an oxide film 14, which are exemplified by a silicon oxidefilm, a silicon nitride film, and a silicon oxide film, respectively. Asdescribed above, since the ONO film 11 is formed only in a region facingthe channel region, the stored charges cannot escape to another region.As a result, it is possible to appropriately transfer electric chargesby aid of the control gate electrodes 30 and the word gate electrodes20.

Each of the side wall insulating films 16 is formed so as to cover sidesurfaces of the control gate electrode 30, the ONO film 11, and thesilicide layer 63 at an opposite side of the word gate electrode 20side. The side wall insulating film 16 is exemplified by a unilaminatesilicon oxide film, or a laminate structure including a silicon oxidefilm, a silicon nitride film, and a silicon oxide film. The respectivegate electrodes 30 of the adjacent memory cells 2 are each surrounded bythe side wall insulating film 16, and an interlayer insulating layer 19,so as to be insulated from each other.

The source/drain diffusion layers 51 are formed at both sides of thechannel region on the surface of the semiconductor substrate 10. Each ofthe source/drain diffusion layers 51 includes a low-concentrationdiffusion layer (LDD diffusion layer) 51 a and a high-concentrationdiffusion layer 51 b. Each of the low-concentration diffusion layers 51a is formed at a position substantially immediately below the side wallinsulating film 16 so as to project from the source/drain diffusionlayer 51 to the channel region. The impurities are exemplified byarsenic (As) or phosphorous (P) in the case of n-type conductivity, andexemplified by boron (B) in the case of p-type conductivity. Each of thesilicide layers 61 is formed on an upper portion of the source/draindiffusion layer 51. The silicide layer 61 is exemplified by cobaltsilicide. An upper portion of each silicide layer 61 is connected with acontact 71, which is further connected to a wiring 72 being an upperlayer (exemplification: bit line).

In the present invention, each of the silicide layers 63 is disposed onthe control gate electrode 30. The silicide layer 63 enables the controlgate electrode 30 to be reduced in resistance. As a result, high speedoperation is enabled in this nonvolatile semiconductor storage device.Also, the top surface of the silicide layer 63 is substantially parallelto the flat surface of the semiconductor substrate 10 and issubstantially flat, and a height of the word gate electrode 20 is higherthan a height of the control gate electrode 30. Accordingly, when thesilicide layer 62 and the silicide layers 63 are formed, it is possibleto provide a sufficient distance between the upper portion of the wordgate electrode 20 and the upper portion of the control gate electrode30. This enables the silicide layer 62 and the silicide layers 63 to beprevented from being short-circuited.

FIG. 11 is a plan view showing the configuration of a nonvolatilesemiconductor storage device according to the first exemplaryembodiment. In FIG. 11, the silicide layers 61, 62, 63, the interlayerinsulating layer 19, and the wiring 72 are omitted.

In the nonvolatile semiconductor storage device, plural memory cells 2(a region surrounded by a broken line) are aligned in a matrix. A wordgate electrode 20 extends in an X-direction, and is shared by the pluralmemory cells 2 aligned in the X-direction. The control gate electrodes30 extend along both sides of the word gate electrode 20 through theword gate insulating films 15 in the X-direction, and are shared by theplural memory cells aligned in the X-direction. The word gate electrode20 and the control gate electrodes 30 also function as wirings.

Also, on the surface of the semiconductor substrate 10 are formed pluraldevice isolation regions 8 that electrically isolate the surface regionsfrom each other, and extend in a Y-direction. The memory cell 2 is aregion that is sandwiched between the device isolation regions 8, andincludes one word gate electrode 20, the control gate electrodes 30 atboth sides of the word gate electrode 20, and regions close to thecontrol gate electrodes 30 (source/drain diffusion layer 51). Forexample, the memory cell is a region surrounded by a square frame(broken line) in the figure. The memory cell 2 shown in FIG. 10corresponds to a cross section taken along a line A-A′ in FIG. 11. Thecontact 71 connects the source/drain diffusion layer 51 of the memorycell 2 to a wiring (not shown, exemplification: bit line) arranged onthe upper layer.

Subsequently, a description will be given of the operation of thenonvolatile semiconductor storage device according to the firstexemplary embodiment with reference to FIG. 10. First, the operation ofwriting information in the memory cell 2 will be described. A positivepotential of about 1 V is applied to the word gate electrode 20, apositive potential of about 6 V is applied to the control gate electrode30 at a writing side (hereinafter referred to as a “select side”), apositive potential of about 3 V is applied to the control gate electrode30 at a non-writing side (hereinafter referred to as an “unselect side”)paired with the control gate electrode 30, a positive potential of about5 V is applied to the source/drain diffusion layer 51 at the selectside, and about 0 V is applied to the source/drain diffusion layer 51 atthe unselect side. As a result, hot electrons that have occurred in thechannel region are implanted into the nitride film 13 of the ONO film 11at the select side. This is called “Channel Hot Electron (CHE)implantation”. As a result, data is written.

Subsequently, a description will be given of the operation of erasinginformation written in the memory cell 2. About 0 V is applied to theword gate electrode 20, a negative potential of about −3 V is applied tothe control gate electrode 30 at the writing side, a positive potentialof about 2 V is applied to the control gate electrode 30 at thenon-writing side, and a positive potential of about 5 V is applied tothe source/drain diffusion layer 51 at the select side. As a result, ahole electron pair occurs due to an inter-band tunnelling, and theholes, or holes having occurred by colliding with those holes, areaccelerated to provide hot holes which are implanted into the nitridefilm 14 of the ONO film 11 at the select side. As a result, negativecharges that have been charged in the nitride film of the ONO film 11are canceled to erase data.

Subsequently, a description will be given of the operation of readinginformation written in the memory cell 2. A positive potential of about2 V is applied to the word gate electrode 20, a positive potential ofabout 2 V is applied to the control gate electrode 30 at the selectside, a positive potential of about 3 V is applied to the control gateelectrode 30 at the unselect side, about 0 V is applied to thesource/drain diffusion layer 51 at the select side, and about 1.5 V isapplied to the source/drain diffusion layer 51 at the unselect side. Inthat state, a threshold value of the memory cell 2 is detected. Whennegative electric charges are stored in the ONO film 11 at the selectside, the threshold value is increased more than that in the case wherethe negative electric charges are not stored. Therefore, the thresholdvalue is detected, thereby making it possible to read information readin the ONO film 11 at the select side. In the memory cell 2 shown inFIG. 10, information of two bits where one bit is provided at each sideof the word gate electrode 20 can be recorded.

Subsequently, a description will be given of a method of manufacturingthe nonvolatile semiconductor storage device according to the firstexemplary embodiment. FIGS. 12 to 28 are cross-sectional views showingrespective processes in the method of manufacturing the nonvolatilesemiconductor storage device according to the first exemplaryembodiment. FIGS. 12 to 28 correspond to a cross section taken along aline A-A′ in FIG. 11. Hereinafter, a description will be given of anexample in which the word gate electrode 20 and the control gateelectrodes 30 are each formed of a polysilicon film.

As shown in FIG. 12, on the surface of the semiconductor substrate 10made of the p-type silicon are laminated a silicon oxide film as theoxide film 12, a silicon nitride film as the nitride film 13, and asilicon oxide film as the oxide film 14 in the stated order. The firstsilicon oxide film 12 is formed in, for example, a thickness of about 3to 5 nm through a wet oxidizing method or a radical oxidizing method.The silicon nitride film 13 is formed in, for example, a thickness ofabout 6 to 10 nm through a chemical vapor deposition (CVD) method. Thelast silicon oxide film 14 is formed in, for example, a thickness ofabout 3 to 10 nm through the radical oxidizing, the wet oxidizing or CVDmethod. As a result, the ONO film 11 functioning as the charge storagelayer is formed. Thereafter, the polysilicon film 30 a is formed so asto cover the ONO film 11 through the CVD method. The polysilicon film 30a is formed in, for example, a thickness of about 50 to 200 nm, anddoped with In-Situ impurities contained in the film or doped withimpurities by ion implantation after the film has been formed. Thepolysilicon film 30 a forms the control gate electrodes 30 in apost-process. Then, the silicon nitride film is formed as the mask film80 through the CVD method so as to cover the polysilicon film 30 a. Themask film 80 is formed in, for example, a thickness of about 50 to 200nm.

Then, as shown in FIG. 13, a photo resist (not shown) having a patternof the word gate electrode 20 is formed through a photolithographytechnique. Then, the mask film 80, the polysilicon film 30 a, and theONO film 11 are sequentially dry-etched with the photo resist as a maskto form a trench 81. The word gate electrode 20 is formed within thetrench 81 in a post-process. Thereafter, the photo resist is removed.

Subsequently, as shown in FIG. 14, the word gate insulating film 15(silicon oxide) is formed through the CVD method so as to cover theinside of the mask film 80 and the trench 81. The word gate insulatingfilm 15 is formed in, for example, a thickness of about 10 to 30 nm. Awidth of the trench 81 after the word gate insulating film 15 has beenformed corresponds to a gate length of the word gate electrode 20. Forexample, the width is about 60 to 200 nm. Then, the polysilicon film 20a is formed through the CVD method so as to cover the word gateinsulating film 15 and fill in the trench 81. The polysilicon film 20 ais formed in, for example, a thickness of about 60 to 200 nm, and thendoped with In-Situ impurities contained in the film or doped withimpurities by ion implantation after the film has been formed. Thepolysilicon film 20 a forms the word gate electrodes 20 in apost-process.

Then, as shown in FIG. 15, the polysilicon film 20 a and the word gateinsulating film 15 are removed and planarized through etch back orchemical mechanical polishing (CMP) so as to be adjusted to the heightof the mask film 80 (remain only within the trench 81). As a result, thepolysilicon film 20 a forms the word gate electrode 20. Subsequently, asshown in FIG. 16, the upper portion of the word gate electrode 20 isthermally oxidized to form the oxide film 82 in, for example, athickness of about 10 to 50 nm. Thereafter, as shown in FIG. 17, themask film 80 is removed by etching. As a result, the upper portion ofthe word gate electrode 20 whose sides are covered with the word gateinsulating film 15 and whose top surface is covered with the oxide film82 projects from the surface of the polysilicon film 30 a.

Subsequently, as shown in FIG. 18, a silicon nitride film is formed as aspacer film 17 a in, for example, a thickness of about 30 to 100 nmthrough the CVD method so as to cover the word gate electrode 20 coveredwith the word gate insulating film 15 and the nitride film 82, and thepolysilicon film 30 a. Thereafter, as shown in FIG. 19, the spacer film17 a is etched back. As a result, a spacer 17 is formed on the sidesurface of the word gate electrode 20 through the word gate insulatingfilm 15, and on the surface of the polysilicon film 30 a. Then, as shownin FIG. 20, the polysilicon film 30 a and the ONO film 11 aresequentially etched with the spacer layer 17 as a mask. As a result, thecontrol gate electrode 30 is formed. The surface of the semiconductorsubstrate 10 is exposed outside of the control gate electrode 30. Atthis time, a width of the spacer layer 17 becomes a gate length of thecontrol gate electrode 30.

Subsequently, as shown in FIG. 21, impurities are implanted into thesurface region of the semiconductor substrate 10 with the spacer layer17, the word gate electrode 20, and the word gate insulating film 15 asa mask to form a low-concentration diffusion layer 51 a. Then, as shownin FIG. 22, a silicon oxide film is formed on the entire surface in 10to 20 nm through the CVD method, and etched back to form a side wallinsulating film 16 on the side surfaces of the spacer layer 17, thecontrol gate electrode 30, and the ONO film 11. Thereafter, as shown inFIG. 23, impurities are implanted into the surface region of thesemiconductor substrate 10 with the side wall insulating film 16, thespacer layer 17, the word gate electrode 20, and the word gateinsulating film 15 as a mask, to form a high-concentration diffusionlayer 51 b. The low-concentration diffusion layer 51 a and thehigh-concentration diffusion layer 51 b constitute the source/draindiffusion layer 51.

Subsequently, a shown in FIG. 24, an oxide film 83 (silicon oxide film)is formed in a thickness of about 3 to 10 nm on the word gate electrode20 and the source/drain diffusion layer 51 through thermal oxidization.Then, as shown in FIG. 25, the spacer layer 17 is selectively removed byetching. Thereafter, as shown in FIG. 26, the oxide film 83 on the wordgate electrode 20 and the source/drain diffusion layer 51 is etched tobe removed.

Then, as shown in FIG. 27, a metal film exemplified by cobalt is formedin a thickness of about 5 to 20 nm on the entire surface through thesputtering method to form the silicide layers 63, 62, and 61, on thecontrol gate electrode 30, the word gate electrode 20, and thesource/drain diffusion layer 51 through a given heat treatment,respectively. Then, as shown in FIG. 28, after the formation of theinterlayer insulating layer 19 on the entire surface, the contact 71that penetrates the interlayer insulating layer 19 is formed on thesource/drain diffusion layer to form a wiring 72 on the contact 71.

With the above operation, the nonvolatile semiconductor storage device(FIG. 10) according to the first exemplary embodiment is manufactured.

In the present invention, the silicide layer 63 is disposed on thecontrol gate electrode 30. The silicide layer 63 enables the controlgate electrode 30 to be reduced in resistance. As a result, the highspeed operation is enabled in this nonvolatile semiconductor storagedevice. Also, the top surface of the silicide layer 63 is substantiallyparallel to the surface of the semiconductor substrate 10 (and thesurface of the control gate electrode 30), and is substantially flat,and spaced apart from the top surface of the word gate electrode 20 witha step. Accordingly, when the silicide layer 62 and the silicide layer63 are formed, it is possible to prevent a layer having such acontinuous electrical conductivity as to short-circuit the silicidelayer 62 and the silicide layer 63 from being formed between the upperportion of the word gate electrode 20 and the upper portion of thecontrol gate electrode 30. Further, a height of the word gate electrode20 is higher than a height of the control gate electrode 30.Accordingly, when the silicide layer 62 and the silicide layer 63 areformed, a sufficient distance can be provided between the upper portionof the word gate electrode 20 and the upper portion of the control gateelectrode 30. As a result, it is possible to effectively prevent thesilicide layer 62 and the silicide layer 63 from being short-circuited.

Also, in the technique of the above JP-A-2004-312009 (FIGS. 1 to 9), aproblem of the controllability of the gate length of the control gatebeing deteriorated has been proved by the inventors' research. FIG. 29is a cross-sectional view showing the problem of the technique ofJP-A-2004-312009. A side surface of a buffer layer 180 is liable to betapered by etching for forming the trench 181 in FIG. 2. That is, aportion of the side surface of the trench 181 at the semiconductorsubstrate 110 side is configured to project toward the trench 181 sideby Δ from a portion of the top surface side. As a result, a side surface117 h of a first insulating spacer 117 is inversely tapered in a firstinsulating spacer 117 formation process shown in FIG. 4. That is, aportion of the side surface 117 h at the semiconductor substrate 110side is configured to be recessed toward the word gate electrode 20 sideby Δ from a portion of the top surface side.

Under the above circumstances, when the first conductive layer 130 a(and the dielectric layer 111) is selectively sequentially removed withthe first insulating spacer 117 as an etching mask in FIG. 8, therearises the following problem. As shown in FIG. 29, the side surface 117h of the first insulating spacer 117 is inversely tapered, and a widthL2 of the first insulating spacer 117 at the side of a bottom surface117 b is shorter than a width L1 at the side of a top surface 117 t byΔ. That is, the top surface 117 t is configured to project from thebottom surface 117 b by Δ. When the first conductive layer 130 a isetched with the above first insulating spacer 117 as a mask, it isconceivable that a width L4 of the first conductive layer 130 a at thetop surface 130 t side varies in a range of from the width L1 of thefirst insulating spacer 117 at the top surface 117 t side to the widthL2 at the bottom surface 117 b side. That is, a gate length of thecontrol gate 130 is varied. The variation becomes larger as a thicknessL3 of the first insulating spacer 117 is thicker. This is becauseetching ions are liable to go around to the inside of the inverse tapermore as a stroke of the etching ions from the top surface 117 t of thefirst insulating spacer 117 to the bottom surface 117 b is longer (athickness L3 is thicker), and the go-around probability is varied.

Besides, the magnitude of Δ of the tapered side surface of the bufferlayer 180 is varied because of the control difficulty. That is, theabove variation can occur when the magnitude of Δ is varied, or whenthere is no variation per se. Correspondingly, in a stage of FIG. 29,the above variation can occur when the magnitude of Δ in the inversetaper is varied, or when there is no variation per se. For that reason,even in this event, in addition to the above case, the width L4 of thefirst conductive layer 130, that is, the gate length of the control gate130 is varied. As a result of occurrence of the above variations, thememory cells are varied, and various characteristics required for write,erase, disturb, and so on cannot be satisfied, and a manufacture yieldis also adversely affected. However, in the nonvolatile semiconductorstorage device and the manufacturing method thereof according to thefirst exemplary embodiment, the above problem can be solved. The reasonis stated below.

FIG. 30 is a cross-sectional view showing the effects of suppressing thetaper in the nonvolatile semiconductor storage device and themanufacturing method thereof according to the first exemplaryembodiment. The figure shows a polysilicon film in processes of FIGS. 19to 20. The spacer layer 17 is formed by etch back in a process of FIG.19. Accordingly, in a height of a top surface 17 t from a bottom surface17 b in the spacer layer 17 (a surface 30 t of the polysilicon film 30a), a height L13 at a side surface 17 h at an opposite side of the wordgate electrode 20 is lower than a height L14 at a side surface 17 k atthe word gate electrode 20 side. That is, the height L13 is lower as theside surface 17 h is farther from the word gate electrode 20. In thissituation, when a flat surface P substantially perpendicular to thesurface of the semiconductor substrate 10 (and the polysilicon film 30a) extends so as to include a line of intersection of the bottom surface17 b and the side surface 17 h, the side surface 17 h is included in theflat surface P substantially perpendicular to the side surface, orinclined toward the word gate electrode 20 side. Accordingly, the sidesurface 17 h is not inclined toward a side apart from the word gateelectrode 20 with respect to the substantially perpendicular flatsurface P. That is, the side surface 17 h has no inverse taper shown inFIG. 29.

When the polysilicon film 30 a is etched with the above spacer layer 17as a mask, since no inverse taper exists, the polysilicon film 30 a isetched by the width of the bottom surface 17 b of the spacer layer 17.As a result, it is possible that a width L12 of the bottom surface 17 bof the spacer layer 17 is made to coincide with a width L15 of the topsurface 30 t of the control gate electrode 30. That is, the width L12 ofthe bottom surface 17 b of the spacer layer 17 can be made to corresponddirectly to the gate length of the control gate electrode 30. In thiscase, since the degree of variation of the width L12 of the bottomsurface 17 b of the spacer layer 17 can be also kept low, a variation inthe gate length of the control gate electrode 30 can be also kept low.That is, it is possible to improve the manufacture yield.

The manufacturing method for the nonvolatile semiconductor storagedevice according to the first exemplary embodiment is clearly differentfrom the manufacturing method disclosed in JP-A-2004-312009 as shown inFIGS. 12 to 28. For that reason, there is no case in which the spacerlayer 17 in the first exemplary embodiment is inversely tapered as shownin FIG. 29, and the above advantage can be obtained. However, in themanufacturing method for the nonvolatile semiconductor storage deviceaccording to the first exemplary embodiment, there is the possibilitythat the upper configurations of the word gate electrode 20 and the gateinsulating film 15 are spread outward as shown in FIG. 30. However, aninfluence of the spread configuration on the top surface 17 t and theside surface 17 h of the spacer layer 17 is extremely small, and therearises no problem in the above effect. The reason will be describedbelow.

FIGS. 31 to 35 are cross-sectional views showing a manufacturing processwhen the upper configuration of the word gate electrode 20 is spread inthe manufacturing method for the nonvolatile semiconductor storagedevice according to the first exemplary embodiment. FIGS. 31 to 35correspond to FIGS. 13, 17, and 18, the mid-flow of FIGS. 18 and 19, andFIG. 19.

As shown in FIG. 31, the mask film 80 has a side surface 80 h liable tobe tapered in etching for forming the trench 81 in FIG. 13. That is, aportion of the side surface 80 h at the top surface side is configuredto be recessed toward the mask film 80 side with respect to a portionthereof at the side of the semiconductor substrate 10 by Δ11. In thatcase, as shown in FIG. 32, the word gate electrode 20 and the word gateinsulating film 15 are configured so that their upper portionconfirmations are spread outward in a stage of FIG. 17. As a result, anupper side wall 15 h of the word gate insulating film 15 is inverselytapered. That is, a portion of the side wall 15 h at the semiconductorsubstrate side is configured to be recessed toward the word gateelectrode 20 side by Δ12 (substantially equal to Δ11) with respect to aportion thereof at the top surface side. In this case, it is assumedthat a height of the inversely tapered portion of the side wall 15 h isL10.

In the above state, when the spacer film 17 a is formed so as to coverthe word gate electrode 20 and the polysilicon film 30 a in a process ofFIG. 18, the spacer film 17 a has its upper configuration spread outwardalong the configurations of the word gate electrode 20 and the word gateinsulating film 15, as shown in FIG. 33. As a result, an upper side wall17 ah of the spacer film 17 a is inversely tapered. That is, a portionof the side wall 17 ah at the side of the semiconductor substrate 10 isconfigured to be recessed toward the word gate electrode 20 side by Δ13with respect to a portion at the top surface side. However, a recess 90being an inversely tapered portion is of no configuration tracing theconfiguration of the upper side wall 15 h of the word gate insulatingfilm 15 as it is, but of a slightly dulled configuration so as to bereduced more than L10 by the thickness of the spacer layer 17 a. Thatis, when it is assumed that a height of an inversely tapered portion ofthe side wall 17 ah is L10A, L10A<L10 as well as Δ13<Δ12 is satisfied.In this way, the formation of the spacer film 17 a whose configurationof the outer surface is free enables an influence of the inverse taperto be suppressed.

Besides, in this state, when the spacer film 17 a is etched back in aprocess of FIG. 18, the spacer film 17 a is etched by etching ions thatare input substantially perpendicularly to the surface of thesemiconductor substrate 10, as shown in FIG. 34. In this situation, thespacer film 17 a is etched in the substantially perpendicular directionwith the substantially same thickness over the entire surface of thesemiconductor substrate 10. As shown in FIG. 34, the spacer film 17 ainitially having a configuration indicated by a broken line is etched inthe substantially perpendicular direction with the substantially samethickness over the entire surface into a configuration indicated by asolid line. In this situation, a recess 90A of the inverse taper isreduced smaller than the recess 90, and a height L10B of the inverselytapered portion is also smaller than the height L10A. Then, when theheight L10A of the inversely tapered portion during the initial phase ofetch back is smaller than the etch back quantity of the spacer film 17a, the inversely tapered portion at the side wall 17 h can be eliminatedfinally, as shown in FIG. 35. Even if the height L10A of the inverselytapered portion during the initial phase of etch back is larger than theetch back quantity of the spacer film 17 a, it is possible to furtherreduce the size of the recess 90A, and the height of the inverselytapered portion can be further reduced more than L10A. As a result, itis possible to remarkably suppress an influence of the inverse taper.

In this way, since the inversely tapered portion is finally eliminatedor remarkably reduced, it is possible to substantially ignore theinversely tapered portion with respect to the intended spacer layer 17.Accordingly, even if the spacer layer 17 is used as an etching mask ofthe polysilicon film 30 a, it is conceivable that etching is hardlyaffected by the inverse taper of the word gate electrode 20. That is,when the polysilicon film 30 a is etched, etching ions reach thepolysilicon film 30 a while their courses are restricted by the topsurface 17 t and the side wall 17 h of the spacer layer 17. In thissituation, because the inversely tapered portion hardly exists, there isno case in which etching ions go around to that portion. Accordingly, itis possible to etch the polysilicon film 30 a with a width predeterminedby the top surface 17 t and the side wall 17 h. Accordingly, it ispossible to form the control gate electrode 30 with the widthpredetermined by the top surface 17 t and the side wall 17 h.

Also, even if the inversely tapered portion remains, and etching ions goaround to that portion for etching, because a width of that portion isextremely small, its influence is extremely small, and the control gateelectrode 30 can be formed with a width that is rarely different fromthe width predetermined by the top surface 17 t and the side wall 17 h.Further, there is a risk that etching ions do not go around to theinversely tapered portion, or go around thereto, thereby causing thewidth of the control gate electrode 30 to be varied. However, asdescribed above, because the height of the inversely tapered portion issmaller than L10A (than L10), the control gate electrode 30 can beformed with a width that is rarely different from the widthpredetermined by the top surface 17 t and the side wall 17 h regardlessof the presence or degree of go-round of etching ions. Accordingly,there arises no problem on a variation in the width of the control gateelectrode 30.

As described above, in the present invention, the height of theinversely tapered portion can be more surely decreased as compared withthe case of JP-A-2004-312009. Further, the film thickness of the spacerlayer 17 a, the height (=L10) of the mask film 80, and the etch backquality are appropriately set, thereby enabling the occurrence of theinversely tapered portion to be finally removed.

Also, in the technique of JP-A-2004-312009, in order to form the controlgate electrode 130, it is necessary to execute selective etching by theaid of at least three kinds of insulating films that can provide etchingselectivity as the buffer layer 180, the first insulating spacer 117,and the gate dielectric layer 115 in the processes of FIGS. 6 and 7.However, it is difficult to select the above insulating films in anormal silicon process. However, in the first exemplary embodiment, inorder to form the control gate electrode 30, for example, it is possibleto execute selective etching by the aid of at least three kinds ofinsulating films that can provide etching selectivity, such as the oxidefilm 82 on the word gate electrode 20, the word gate insulating film 15(silicon oxide), and the spacer layer 17 (silicon oxide) in theprocesses of FIGS. 19 and 20. That is, the first exemplary embodimentcan be readily realized in a normal silicon process.

Second Exemplary Embodiment

First, a description will be given of the configuration of a nonvolatilesemiconductor storage device according to a second exemplary embodimentof the present invention. FIG. 36 is a cross-sectional view showing theconfiguration of the nonvolatile semiconductor storage device accordingto the second exemplary embodiment of the present invention. FIG. 36exemplifies a flash memory cell with a MONOS structure as a memory cell2 a of the nonvolatile semiconductor storage device according to thesecond exemplary embodiment.

The memory cell 2 a differs from that in the first exemplary embodimentin that the control gate electrode 30 is disposed on only one side ofthe word gate electrode 20. Other configurations (including FIG. 11) andoperation are identical with those in the first exemplary embodiment,and therefore their description will be omitted.

Subsequently, a description will be given of a method of manufacturingthe nonvolatile semiconductor storage device according to the secondexemplary embodiment. FIGS. 37 to 47 are cross-sectional views showingrespective processes in the method of manufacturing the nonvolatilesemiconductor storage device according to the second exemplaryembodiment. FIGS. 37 to 47 correspond to a cross section taken along aline A-A′ in FIG. 11. Hereinafter, a description will be given of anexample in which the word gate electrode 20 and the control gateelectrodes 30 are each formed of a polysilicon film.

Since an initial process in the method of manufacturing the nonvolatilesemiconductor storage device according to the second exemplaryembodiment is identical with the processes of FIGS. 12 to 19 in thefirst exemplary embodiment, its description will be omitted. A result ofthose processes becomes a state shown in FIG. 37 (as in FIG. 19).

Subsequently, as shown in FIG. 38, a photo resist 88 that covers onlyone side of the word gate electrode 20 is formed through thephotolithography technique. Then, the spacer layer 17 at one side of theword gate electrode 20 is removed with the photo resist 88 as a mask.Thereafter, the photo resist 88 is removed.

Subsequently, as shown in FIG. 39, the polysilicon film 30 a and the ONOfilm 11 are sequentially etched with the spacer layer 17 as a mask. As aresult, the control gate electrode 30 is formed at one side of the wordgate electrode 20. The surface of the semiconductor substrate 10 isexposed outside of the control gate electrode 30 and at one side of theword gate electrode 20. In this case, the width of the spacer layer 17becomes the gate length of the control gate electrode 30.

Subsequently, as shown in FIG. 40, impurities are implanted into thesurface region of the semiconductor substrate 10 with the spacer layer17, the word gate electrode 20, and the word gate insulating film 15 asa mask to form a low-concentration diffusion layer 51 a. Then, as shownin FIG. 41, a silicon oxide film is formed on the entire surface throughthe CVD method, and etched back to form a side wall insulating film 16on the side surfaces of the spacer layer 17, the control gate electrode30, and the ONO film 11. Thereafter, as shown in FIG. 42, impurities areimplanted into the surface region of the semiconductor substrate 10 withthe side wall insulating film 16, the spacer layer 17, the word gateelectrode 20, and the word gate insulating film 15 as a mask, to form ahigh-concentration diffusion layer 51 b. The low-concentration diffusionlayer 51 a and the high-concentration diffusion layer 51 b constitutethe source/drain diffusion layer 51.

Subsequently, a shown in FIG. 43, an oxide film 84 (silicon oxide film)is formed in a thickness of about 3 to 10 nm on the word gate electrode20 and the source/drain diffusion layer 51 through thermal oxidization.Then, as shown in FIG. 44, the spacer layer 17 is selectively removed byetching. Thereafter, as shown in FIG. 45, the oxide film 84 on the wordgate electrode 20 and the source/drain diffusion layer 51 is etched tobe removed.

Then, as shown in FIG. 46, a metal film exemplified by cobalt is formedon the entire surface to form the silicide layers 63, 62, and 61, on thecontrol gate electrode 30, the word gate electrode 20, and thesource/drain diffusion layer 51 through a given heat treatment,respectively. Then, as shown in FIG. 47, after the formation of theinterlayer insulating layer 19 on the entire surface, the contact 71that penetrates the interlayer insulating layer 19 is formed on thesource/drain diffusion layer 51 to form a wiring 72 on the contact 71.

With the above operation, the nonvolatile semiconductor storage device(FIG. 36) according to the second exemplary embodiment is manufactured.

Similarly, in the second exemplary embodiment, the same advantages asthose in the first exemplary embodiment can be obtained. Also, in thesecond exemplary embodiment, since there is applied a configuration of 1bit/1 cell system suitable for high speed operation in which the controlgate electrode 30 is disposed on only one side of the word gateelectrode 20, the high speed operation and the miniaturization of cellsize are enabled.

Although the invention has been described above in connection withseveral exemplary embodiments thereof, it will be appreciated by thoseskilled in the art that those embodiments are provided solely forillustrating the invention, and should not be relied upon to construethe appended claims in a limiting sense.

Further, it is noted that, notwithstanding any claim amendments madehereafter, applicant's intent is to encompass equivalents all claimelements, even if amended later during prosecution.

It is apparent to one skilled in the art that the present invention maybe changed or modified without departing from the spirit and scope ofthe apparatus claims that are indicated in the subsequent pages as wellas methods that are indicated below.

AA. A method for manufacturing a nonvolatile semiconductor storagedevice, comprising:

sequentially forming a charge storage layer, a conductive layer, and amask layer above a semiconductor substrate;

sequentially removing the mask layer, the conductive layer, and thecharge storage layer to form a trench;

covering an inside of the trench with an insulating layer;

forming a word gate so as to fill in the trench whose inside is coveredwith the insulating layer;

removing the mask layer;

forming a spacer layer so as to cover the conductive layer and the wordgate;

etching the spacer layer to form a sidewall spacer above a side of theword gate via the insulating layer;

removing the conductive layer and the charge storage layer with thesidewall spacer as a mask to form a control gate;

removing the sidewall spacer; and

siliciding upper portions of the word gate and the control gate.

BB. The method according to method AA, wherein the charge storage layerincludes a laminated layer of a first silicon oxide layer, a siliconnitride layer and a second silicon oxide layer.

CC. The method according to method AA, further comprising removing oneof the spacer layers formed on both sides of the word gate before theremoving of the conductive layer and the charge storage layer.

DD. The method according to method AA, further comprising implanting anion to form a first diffusion layer and a second diffusion layer on thesemiconductor substrate,

wherein the charge storage layer and the word gate are disposed betweenthe first diffusion layer and the second diffusion layer,

wherein the siliciding comprises siliciding upper portions of the firstdiffusion layer and the second diffusion layer.

EE. The method according to method AA, wherein a top surface of thesilicide layer is substantially flat.

1. A nonvolatile semiconductor storage device, comprising: asemiconductor substrate; a charge storage layer formed above thesemiconductor substrate; a control gate formed above the charge storagelayer; a silicide layer formed above the control gate; and a word gateformed above a side of the control gate, wherein a top surface of thesilicide layer is substantially flat.
 2. The nonvolatile semiconductorstorage device according to claim 1, wherein the silicide layercomprises a first silicide layer, the nonvolatile semiconductor storagedevice further comprising a second silicide layer formed on the wordgate.
 3. The nonvolatile semiconductor storage device according to claim1, wherein a height of the word gate is higher than a height of thecontrol gate.
 4. The nonvolatile semiconductor storage device accordingto claim 1, wherein the charge storage layer includes: a first siliconoxide layer; a silicon nitride layer formed above the first siliconoxide layer; and a second silicon oxide layer formed above the siliconnitride layer.
 5. The nonvolatile semiconductor storage device accordingto claim 1, further comprising: a first diffusion layer formed on thesemiconductor substrate; and a second diffusion layer formed on thesemiconductor substrate, wherein the charge storage layer and the wordgate are disposed between the first diffusion layer and the seconddiffusion layer.
 6. The nonvolatile semiconductor storage deviceaccording to claim 5, wherein the silicide layer comprises a firstsilicide layer, the nonvolatile semiconductor storage device furthercomprising: a second silicide layer formed on the word gate; a thirdsilicide layer formed on the first diffusion layer; and a fourthsilicide layer formed on the second diffusion layer.
 7. A nonvolatilesemiconductor storage device, comprising: a semiconductor substrate; afirst charge storage layer formed above the semiconductor substrate; asecond charge storage layer formed above the semiconductor substrate; aword gate formed between the first charge storage layer and the secondcharge storage layer; a first control gate formed above the first chargestorage layer; a second control gate formed above the second chargestorage layer; a first silicide layer formed above the first controlgate; and a second silicide layer formed above the second control gate,wherein a top surface of the silicide layer is substantially flat. 8.The nonvolatile semiconductor storage device according to claim 7,wherein the first charge layer and the second charge layer are disposedsymmetrically with respect to the word gate, wherein the first controlgate and the second control gate are disposed symmetrically with respectto the word gate, wherein the first silicide layer and the secondsilicide layer are disposed symmetrically with respect to the word gate.9. The nonvolatile semiconductor storage device according to claim 7,further comprising a third silicide layer formed on the word gate. 10.The nonvolatile semiconductor storage device according to claim 7,wherein a height of the word gate is higher than a height of the firstcontrol gate and the second control gate.
 11. The nonvolatilesemiconductor storage device according to claim 7, wherein the firstcharge storage layer includes: a first silicon oxide layer; a firstsilicon nitride layer formed above the first silicon oxide layer; and asecond silicon oxide layer formed above the first silicon nitride layer,wherein the second charge storage layer includes: a third silicon oxidelayer; a second silicon nitride layer formed above the third siliconoxide layer; and a fourth silicon oxide layer formed above the secondsilicon nitride layer.
 12. The nonvolatile semiconductor storage deviceaccording to claim 7, further comprising: a first diffusion layer formedon the semiconductor substrate; and a second diffusion layer formed onthe semiconductor substrate, wherein the first charge storage layer, thesecond charge storage layer and the word gate are disposed between thefirst diffusion layer and the second diffusion layer.
 13. Thenonvolatile semiconductor storage device according to claim 12, furthercomprising: a third silicide layer formed on the word gate; a fourthsilicide layer formed on the first diffusion layer; and a fifth silicidelayer formed on the second diffusion layer.